Dielectric Relaxation of Cationic Surfactants in Aqueous Solution. 1. Solvent Relaxation

Abstract

Complex dielec. permittivity spectra, ̂ε(ν), in the frequency range 0.09 ≤ ν/GHz ≤ 89 were recorded for aq. solns. of octyl-, dodecyl-, and hexadecyltrimethylammonium bromide and dodecyltrimethylammonium chloride at room temp. and concns. c ≤ 1 mol dm-3. Addnl., for selected solns. the temp. dependence of ̂ε(ν) was examd. in the range of 0 ≤ θ/°C ≤ 65 to obtain Eyring activation enthalpies and entropies of the relaxation times. A superposition of up to 5 Debye relaxation processes is required for a consistent fit of all spectra. The low-frequency dispersion steps with relaxation times τ1 ≈ 1.5 ns and τ2 ≈ 300 ps can be assigned to the diffusion of free and bound counterions surrounding the charged micelles. The remaining relaxation processes are due to the solvent. As in the pure solvent, a small contribution of fast mobile H2O mols., τ5 ≈ 1 ps, can be obsd. besides the dominating relaxation of the hydrogen bond network of bulk H2O, τ4 ≈ 8 ps. Addnl., a slower relaxation process, τ3 ≈ 25 ps, is detected which can be attributed to H2O mols. within the micelles and in the vicinity of the hydrophobic tail of monomeric octyltrimethylammonium ions. From the amplitudes of the H2O relaxation processes hydration nos. of the surfactant ions and micelles as well as the vol. fraction of micelles were calcd.